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          JavaScript手写数组常用函数
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        <p>在代码开发过程中，我们常常使用数组的一些 原生<code>api</code>进行操作，大大方便了我们的操作。其中包含 <code>forEach</code>、<code>filter</code>、<code>find</code>、<code>findIndex</code>、<code>map</code>、<code>some</code>、<code>every</code>、<code>reduce</code>等函数方法，我们能不能自己写出这些方法。为了统一代码，我在前面增加了参数说明，看了说明之后代码可谓一目了然</p>
<span id="more"></span>

<hr>
<h3 id="参数说明"><a href="#参数说明" class="headerlink" title="参数说明"></a>参数说明</h3><ul>
<li>callback 回调函数</li>
<li>context 执行 callback时使用的 this 值</li>
<li>current 数组中正在处理的元素</li>
<li>index 当前索引</li>
<li>array 源数组</li>
<li>accumulator 累加器</li>
<li>initialValue reduce或者reduceRight 第一次调用 callbackFn 函数时的第一个参数的值默认值</li>
<li>self 自己实现的 this 对象</li>
</ul>
<h3 id="1-forEach-函数"><a href="#1-forEach-函数" class="headerlink" title="1.forEach 函数"></a>1.forEach 函数</h3><p><strong>语法：</strong> <code>arr.forEach(callback(current [, index [, array]])[, context])</code></p>
<p><strong>方法功能：</strong>回调参数为：每一项、索引、原数组, 对数组的每个元素执行一次给定的函数。</p>
<p><strong>返回：</strong> undefined。</p>
<p>自定义函数：myForEach。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">Array.prototype.myForEach &#x3D; function(callback, context) &#123;</span><br><span class="line">    if (typeof callback !&#x3D;&#x3D; &#39;function&#39;) throw (&#39;callback参数必须是函数&#39;);</span><br><span class="line">    let self &#x3D; this,</span><br><span class="line">        len &#x3D; self &amp;&amp; element.length || 0;</span><br><span class="line">    if (!context) context &#x3D; self;</span><br><span class="line">    for (let index &#x3D; 0; index &lt; len; index++) &#123;</span><br><span class="line">        callback.call(context, self[index], index, self);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;;</span><br></pre></td></tr></table></figure>

<h3 id="2-filter-函数"><a href="#2-filter-函数" class="headerlink" title="2.filter 函数"></a>2.filter 函数</h3><p><strong>语法：</strong> <code>var newArray = arr.filter(callback(current[, index[, array]])[, context])</code></p>
<p><strong>方法功能：</strong> 创建一个新数组, 过滤掉回调函数返回值不为true的项,其包含通过所提供函数实现的测试的所有元素。</p>
<p><strong>返回：</strong> 一个新的、由通过测试的元素组成的数组，如果没有任何数组元素通过测试，则返回空数组。</p>
<p>自定义函数：myFilter。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line">Array.prototype.myFilter &#x3D; function(callback, context) &#123;</span><br><span class="line">    if (typeof callback !&#x3D;&#x3D; &#39;function&#39;) throw (&#39;callback参数必须是函数&#39;);</span><br><span class="line">    let self &#x3D; this,</span><br><span class="line">        len &#x3D; self &amp;&amp; self.length || 0,</span><br><span class="line">        newArray &#x3D; [];</span><br><span class="line">    if (!context) context &#x3D; self;</span><br><span class="line">    for (let index &#x3D; 0; index &lt; len; index++) &#123;</span><br><span class="line">        if (callback.call(context, self[index], index, self)) newArray.push(self[index]);</span><br><span class="line">    &#125;</span><br><span class="line">    return newArray;</span><br><span class="line">&#125;;</span><br></pre></td></tr></table></figure>

<h3 id="3-find-函数"><a href="#3-find-函数" class="headerlink" title="3.find 函数"></a>3.find 函数</h3><p><strong>语法：</strong><code>arr.find(callback[, context])</code></p>
<p><strong>方法功能：</strong> 返回数组中满足提供的测试函数的第一个元素的值。否则返回 undefined。</p>
<p><strong>返回：</strong> 数组中第一个满足所提供测试函数的元素的值，否则返回 undefined。</p>
<p>自定义函数：myFind。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line">Array.prototype.myFind &#x3D; function(callback, context) &#123;</span><br><span class="line">    if (typeof callback !&#x3D;&#x3D; &#39;function&#39;) throw (&#39;callback参数必须是函数&#39;);</span><br><span class="line">    let self &#x3D; this,</span><br><span class="line">        len &#x3D; self &amp;&amp; self.length || 0;</span><br><span class="line">    if (!context) context &#x3D; self;</span><br><span class="line">    for (let index &#x3D; 0; index &lt; len; index++) &#123;</span><br><span class="line">        if (callback.call(context, self[index], index, self)) &#123;</span><br><span class="line">            return self[index];</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">    return undefined;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h3 id="4-findIndex-函数"><a href="#4-findIndex-函数" class="headerlink" title="4.findIndex 函数"></a>4.findIndex 函数</h3><p><strong>语法：</strong> <code>arr.findIndex(callback[, context])</code></p>
<p><strong>方法功能：</strong> 返回数组中满足提供的测试函数的第一个元素的索引。否则返回 -1。</p>
<p><strong>返回：</strong> 数组中通过提供测试函数的第一个元素的索引。否则，返回-1。</p>
<p>自定义函数：myFindIndex。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line">Array.prototype.myFindIndex &#x3D; function(callback, context) &#123;</span><br><span class="line">    if (typeof callback !&#x3D;&#x3D; &#39;function&#39;) throw (&#39;callback参数必须是函数&#39;);</span><br><span class="line">    let self &#x3D; this,</span><br><span class="line">        len &#x3D; self &amp;&amp; self.length || 0;</span><br><span class="line">    if (!context) context &#x3D; self;</span><br><span class="line">    for (let index &#x3D; 0; index &lt; len; index++) &#123;</span><br><span class="line">        if (callback.call(context, self[index], index, self)) return index;</span><br><span class="line">    &#125;</span><br><span class="line">    return -1;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h3 id="5-fill函数"><a href="#5-fill函数" class="headerlink" title="5.fill函数"></a>5.fill函数</h3><p><strong>语法：</strong> <code>arr.fill(value[, start[, end]])</code></p>
<p><strong>方法功能：</strong> 用一个固定值填充一个数组中从起始索引到终止索引内的全部元素。不包括终止索引。</p>
<p><strong>返回：</strong> 返回替换的值，原数组发生改变。</p>
<p>自定义函数：myFill。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line">Array.prototype.myFill &#x3D; function(value, start &#x3D; 0, end) &#123;</span><br><span class="line">    let self &#x3D; this,</span><br><span class="line">        len &#x3D; self &amp;&amp; self.length || 0;</span><br><span class="line">    end &#x3D; end || len;</span><br><span class="line">    let loopStart &#x3D; start &lt; 0 ? 0 : start, &#x2F;&#x2F; 设置循环开始值</span><br><span class="line">        loopEnd &#x3D; end &gt;&#x3D; len ? len : end; &#x2F;&#x2F; 设置循环结束值</span><br><span class="line"></span><br><span class="line">    for (; loopStart &lt; loopEnd; loopStart++) &#123;</span><br><span class="line">        self[loopStart] &#x3D; value;</span><br><span class="line">    &#125;</span><br><span class="line">    return self;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h3 id="6-map-函数"><a href="#6-map-函数" class="headerlink" title="6.map 函数"></a>6.map 函数</h3><p><strong>语法：</strong> <code>var newArray = arr.map(function callback(current[, index[, array]]) &#123;// Return self for newArray &#125;[, context])</code></p>
<p><strong>方法功能：</strong> 创建一个新数组，其结果是该数组中的每个元素是调用一次提供的函数后的返回值。</p>
<p><strong>返回：</strong> 测试数组中是不是至少有1个元素通过了被提供的函数测试。它返回的是一个Boolean类型的值。 一个由原数组每个元素执行回调函数的结果组成的新数组。</p>
<p>自定义函数：myMap。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line">Array.prototype.myMap &#x3D; function(callback, context) &#123;</span><br><span class="line">    if (typeof callback !&#x3D;&#x3D; &#39;function&#39;) throw (&#39;callback参数必须是函数&#39;);</span><br><span class="line">    let self &#x3D; this,</span><br><span class="line">        len &#x3D; self &amp;&amp; self.length || 0,</span><br><span class="line">        result &#x3D; [];</span><br><span class="line">    if (!context) context &#x3D; self;</span><br><span class="line">    for (let index &#x3D; 0; index &lt; len; index++) &#123;</span><br><span class="line">        result[index] &#x3D; callback.call(context, self[index], index, self);</span><br><span class="line">    &#125;</span><br><span class="line">    return result;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h3 id="7-some-函数"><a href="#7-some-函数" class="headerlink" title="7.some 函数"></a>7.some 函数</h3><p><strong>语法：</strong> <code>arr.some(callback(current[, index[, array]])[, context])</code></p>
<p><strong>方法功能：</strong> 测试数组中是不是至少有1个元素通过了被提供的函数测试,回调函数返回值一个为true 结果就为true, 否则为false。它返回的是一个Boolean类型的值。</p>
<p><strong>返回：</strong> 数组中有至少一个元素通过回调函数的测试就会返回true；所有元素都没有通过回调函数的测试返回值才会为false。</p>
<p>自定义函数：mySome。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line">Array.prototype.mySome &#x3D; function(callback, context) &#123;</span><br><span class="line">    if (typeof callback !&#x3D;&#x3D; &#39;function&#39;) throw (&#39;callback参数必须是函数&#39;);</span><br><span class="line">    let self &#x3D; this,</span><br><span class="line">        len &#x3D; self &amp;&amp; self.length || 0;</span><br><span class="line">    if (!context) context &#x3D; self;</span><br><span class="line">    for (let index &#x3D; 0; index &lt; len; index++) &#123;</span><br><span class="line">        if (callback.call(context, self[index], index, self)) return true;</span><br><span class="line">    &#125;</span><br><span class="line">    return false;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h3 id="8-every-函数"><a href="#8-every-函数" class="headerlink" title="8.every 函数"></a>8.every 函数</h3><p><strong>语法：</strong> <code>arr.every(callback(current[, index[, array]])[, context])</code></p>
<p><strong>方法功能</strong>：测试一个数组内的所有元素是否都能通过某个指定函数的测试,所有回调函数返回值都为true时 结果为true,否则为false。它返回一个布尔值。</p>
<p><strong>返回：</strong> 如果回调函数的每一次返回都为 true 值，返回 true，否则返回 false。</p>
<p>自定义函数：myEvery。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line">Array.prototype.myEvery &#x3D; function(callback, context) &#123;</span><br><span class="line">    if (typeof callback !&#x3D;&#x3D; &#39;function&#39;) throw (&#39;callback参数必须是函数&#39;);</span><br><span class="line">    let self &#x3D; this,</span><br><span class="line">        len &#x3D; self &amp;&amp; self.length || 0;</span><br><span class="line">    if (!context) context &#x3D; self;</span><br><span class="line">    for(let index &#x3D; 0; index &lt; len; index++) &#123;</span><br><span class="line">        if (!callback.call(context, element[index], index, element)) return false;</span><br><span class="line">    &#125;</span><br><span class="line">    return true;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h3 id="9-reduce-函数"><a href="#9-reduce-函数" class="headerlink" title="9.reduce 函数"></a>9.reduce 函数</h3><p><strong>语法：</strong><code>arr.reduce(callback(accumulator, current[, index[, array]])[, initialValue])</code></p>
<p><strong>方法功能：</strong>对数组中的每个元素执行一个由您提供的reducer函数(升序执行)，将其结果汇总为单个返回值。相比其他方法多了一个参数即上次调用的返回值，<br>最后一个回调函数的返回值为reduce的结果，可以指定累积的初始值，不指定初始值从第二项开始遍历</p>
<p><strong>返回：</strong> 函数累计处理的结果。</p>
<p>自定义函数：myReduce。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">Array.prototype.myReduce &#x3D; function(callback, initialValue) &#123;</span><br><span class="line">    if (typeof callback !&#x3D;&#x3D; &#39;function&#39;) throw (&#39;callback参数必须是函数&#39;);</span><br><span class="line">    let self &#x3D; this,</span><br><span class="line">        len &#x3D; self.length || 0;</span><br><span class="line">    let result &#x3D; initialValue ? initialValue : self[0]; &#x2F;&#x2F; 不传默认取数组第一项</span><br><span class="line">  	let index &#x3D; initialValue ? 0 : 1;</span><br><span class="line"></span><br><span class="line">    while (index &lt; len) &#123;</span><br><span class="line">        if (index in self) result &#x3D; callback(result, self[index], index, self);</span><br><span class="line">        index++;</span><br><span class="line">    &#125;</span><br><span class="line">    return result;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

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